Add FlagManager instance to SF.

Bug: 190769260
Test: make;

1. Examine the value returned by FlagManager:
   `adb shell dumpsys SurfaceFlinger | grep -A 10 "FlagManager"`
2. Examine the value returned by P/H:
   `adb shell device_config get surface_flinger_native_boot
   DemoFeature__demo_flag`

Change-Id: Ied375fa732c33be31cc5e1c6dcdfb024aecb3e0d